The tragic murder of a British man in Libya should be investigated thoroughly by authorities in the north African country, the Foreign Office has said.

The British man and a woman from New Zealand were shot dead near the coastal area of Mellitah in the west of the country, according to a Libyan security source.

In a statement released tonight, the Foreign Office said it was calling upon the Libyan government to carry out a thorough investigation into the incident.

"We are deeply saddened by the murder of a British national and New Zealander in Libya," it said.

"Police have informed their families and consular officials are providing full consular support in Libya and the UK. Our thoughts are with them at this difficult time.

"Our charge d'affaires has raised the shooting with the Libyan authorities and we are liaising closely with them on follow-up.

"We call upon the Libyan government to carry out a thorough investigation into this tragic incident and to continue to do all it can to bring to justice the perpetrators of this appalling crime, as it strives to build strong rule of law in Libya."

A photograph which is claimed to be of the dead pair has been posted on Facebook.

The grim picture, which was taken at night, shows them both lying face down on sand.

Near them is what appears to be a picnic blanket with their belongings spread out on it, alongside a backpack.

Sources said the British man was not a teacher, contrary to reports.

Mellitah is near the town of Zuwarah and 60 miles west of Tripoli.

The area is home to a large oil and gas complex co-owned by Italian company ENI.

There were also claims that the pair were workers at Mellitah Oil and Gas, which ranks itself as the biggest oil company in Libya.

No-one at the company was available for comment.

The killings come a month after a US teacher was shot dead in the eastern city of Benghazi, after which the Foreign Office said further attacks against Westerners were likely.